Which of the following factors primarily affects the distribution of organisms in an ecosystem?

Distribution means where organisms are found in an ecosystem. Temperature is treated as the main controlling factor because it decides where life can survive at all. Every living thing depends on enzymes, and enzymes only work well inside a certain range of temperature. Too cold and reactions become too slow; too hot and the enzymes are denatured and the organism dies.

Temperature is also the factor that changes most across the earth — from hot rainforest and desert to cool highlands and polar regions — which is why the big world zones of vegetation and animals follow temperature bands. It also affects rainfall, humidity and how much oxygen dissolves in water, so it controls other factors too.

The other choices matter, but they act on a smaller scale. Day length mainly affects timing, such as when plants flower or when birds migrate, not where life can exist. Soil pH affects plants and soil organisms in one patch of ground, and it does not apply to water or air habitats. Wind speed mainly changes water loss and seed spreading in exposed places. Temperature is the one that limits organisms everywhere.